Drop trailing slash of prefix in #get_local_files

The bucket setup in `AssetSync::Storage#bucket` uses fuzzy matching when
`config.prefix` is present. This can present a problem in some cases, as
it doesn't allow for distinguishing between (e.g.) a bucket folder
called `assets/` and another folder called `assets-temp/`. A situation
could arise where the latter folder has thousands/millions of files and
everything slows to a crawl while we wait for the bucket object to
initialize.

This change allows developers to be more specific in their
`config.prefix` by using a trailing slash for their folder name.

lib/asset_sync/storage.rb

@@ -140,7 +140,7 @@ def get_local_files
 
       log "Using: Directory Search of #{path}/#{self.config.assets_prefix}"
       Dir.chdir(path) do
-        to_load = self.config.assets_prefix.present? ? "#{self.config.assets_prefix}/**/**" : '**/**'
+        to_load = self.config.assets_prefix.present? ? File.join(self.config.assets_prefix, '/**/**') : '**/**'
         Dir[to_load]
       end
     end